The field of the invention is magnetic resonance imaging (MRI) and in particular decoupling circuits for local coils for use in receiving MRI signals.
In MRI, a uniform magnetic field B0 is applied to an imaged object along the z-axis of a Cartesian coordinate system, the origin of which is approximately centered within the imaged object. The effect of the magnetic field B0 is to align the object""s nuclear spins along the z-axis.
In response to a radio frequency (RF) excitation signal of the proper frequency, oriented within the x-y plane, the nuclei precess about the z-axis at their Larmor frequencies according to the following equation:
xcfx89=xcex3B0
where xcfx89 is the Larmor frequency, and xcex3 is the gyromagnetic ratio which is constant and a property of the particular nuclei.
Water, because of its relative abundance in biological tissue and the properties of its nuclei, is of principle concern in such imaging. The value of the gyromagnetic ratio xcex3 for water is 42.6 MHz/Tesla and therefore in a 1.5 Tesla polarizing magnetic field B0 the resonant or Larmor frequency of water is approximately 63.9 MHz.
In a typical imaging sequence, the RF excitation signal is centered at the Larmor frequency (o and applied to the imaged object at the same time as a magnetic field gradient Gz is applied. The gradient field Gz causes only the nuclei in a slice through the object along an x-y plane to have the resonant frequency xcfx89 and to be excited into resonance.
After the excitation of the nuclei in this slice, magnetic field gradients are applied along the x and y axes. The gradient along the x-axis, Gx, causes the nuclei to precess at different frequencies depending on their position along the x-axis, that is, Gx spatially encodes the precessing nuclei by frequency. The y axis gradient, Gy, is incremented through a series of values and encodes y position into the rate of change of phase of the processing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by the local coil and recorded as an NMR signal. From this NMR signal, a slice image may be derived according to well-known reconstruction techniques. An overview NMR image reconstruction is contained in the book xe2x80x9cMagnetic Resonance Imaging, Principles and Applicationsxe2x80x9d by D. N. Kean and M. A. Smith.
The quality of the image produced by MRI techniques is dependent, in part, on the strength of the NMR signal received from the precessing nuclei. For this reason, it is known to use an independent RF receiving coil placed in close proximity to the region of interest of the imaged object to improve the strength of this received signal. Such coils are termed xe2x80x9clocal coilsxe2x80x9d or xe2x80x9clocal coilsxe2x80x9d. The smaller area of the local coil permits it to accurately focus on NMR signal from the region of interest. The local coils are tuned with capacitors placed in series with the distributed inductance of the coil conductors to create a series resonance near the NMR frequency that helps reject noise signals of other frequencies.
A major technical problem in NMR systems is xe2x80x9cdecouplingxe2x80x9d the local coil from the RF excitation signal from the transmit coil during stimulation of the magnetic resonance. Such decoupling reduces the distortion of the excitation field by the local coil and prevents potential damage to the sensitive circuits connected to the local coil from possibly large induced voltages. Further, decoupling prevents high current flow in the local coil such as may cause damage or heating of the local coil.
One method of decoupling the local coil from the RF excitation field is through the use of one or more diodes used as solid state switches and positioned along the local coil to be activated by an external electrical signal before the application of the RF excitation field itself. In one such xe2x80x9cactive decouplingxe2x80x9d technique, series connected pin diodes and inductors are placed in parallel across the tuning capacitors with the inductors sized to create a parallel resonance at the NMR frequency when the diodes are conducting. As is understood in the art, the created parallel resonance blocks current flow at that diode.
Alternatively, it may be desirable to also employ xe2x80x9cpassive decouplingxe2x80x9d that does not require the application of a direct current to the diodes from an external source. In passive decoupling, parallel back-to-back diodes may be used to produce a circuit element that conducts at voltages above the level of the NMR signal but below the level of the RF excitation signal.
In a combined active and passive decoupling system, these back-to-back diodes are placed in parallel with the pin diode to activate at times when the pin diode is not conducting and thus, with the pin diode, to provide both active and passive decoupling. A direct-current blocking capacitor is placed in series with the back-to-back diodes to block the direct current used to activate the pin diodes.
New MRI techniques require repeated RF excitation at a high repetition rate. With these techniques, it is important that the coil be returned to a normal state as soon as possible after decoupling. Unfortunately, slow recovery time, particularly of the combined active and passive decoupling circuits, limits the speed at which NMR signals may be acquired.
The present inventors have determined that the recovery time of the combined active and passive decoupling circuit can be significantly improved by providing a discharge path shunting the passive decoupling circuit. Although the inventors do not wish to be bound by a particular theory, it is believed that the discharge path provides an alternative, lower impendence discharge path, than the diodes of the passive decoupling circuit. The discharge path allows discharge of the charge accumulated, for example, on the blocking capacitor used to separate the active and passive components. By discharging this energy faster, the diodes are returned more quickly to a non-conducting state suitable for detection of an NMR signal.
Specifically, the present invention provides a circuit for decoupling a local coil used in a magnetic resonance system where the local coil includes at least one series capacitor. The circuit provides a decoupling inductor sized to create a parallel resonance at an NMR frequency when connected in parallel with the series capacitor. A passive decoupling circuit connects the decoupling inductor in parallel with the series capacitor upon the occurrence of an RF excitation signal. A discharge circuit is provided to discharge the passive decoupling circuit after conclusion of the RF excitation signal.
It is thus is one object of the invention to provide faster recovery of a passive decoupling circuits to allow faster acquisition of a series of NMR signals enabling the use of local coils with a variety of new MRI techniques.
The passive decoupling circuit may be connected in series with the decoupling inductor and the series combination of the passive decoupling circuit and the decoupling inductor may be placed in parallel with the series capacitance.
Thus, it is another object of the invention to provide a system that works with well-known passive decoupling circuits.
The passive decoupling circuit may include a capacitor connected in series with first and second diodes in parallel back-to-back configuration.
Thus, it is another object of the invention to provide a circuit that discharges the blocking capacitor used when a passive decoupling circuit is combined with active decoupling.
The discharge circuit may be a discharge inductor coupled in parallel across the first and second diode.
Thus, it is another object of the invention to provide a discharge path that compliments the diodes in the circuit (that provide decreased discharge current flow with time) the inductor providing an increasing discharge current flow with time per normal inductor characteristics.
The local coil may be any type of MRI local coil having series capacitances and requiring decoupling including local coils, birdcage coils, quadrature coils and phased array coils.
Thus, it is another object of the invention to provide improved imaging speed to a variety of coil types.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.